The present state of the art for delivering substances to the breathing passages of a subject in an aerosol form includes either formulation in and delivery from an aerosol container comprising a mixture of the substance to be delivered and a compressed gas, or a nebulizer, which utilizes a compressed gas from an outside source to carry the nebulized aerosol to the subject.
A system for delivering a metered dose of a nebulized antigen has been described and claimed by Rosenthal and French, U.S. Pat. No. 4,106,503. This system relies upon control of compressed air fed into a nebulizer to obtain a metered dose and includes a detector for sensing initiation of subject inhalation. It is not believed that a conventional nebulizer such as that described in U.S. Pat. No. 4,106,503 permits administration of a closely controlled dose of the substance since the nebulizer of such an apparatus is not given the opportunity to reach steady state operation prior to administration of a dose of the substance. That is, gas flows through the nebulizer only during actual administration of a dose. Upon initiation of gas flow, it is believed that substantial time is required before the size of the aerosolized droplets of the solution or suspension of the substance become essentially constant and to therefore permit administration of a closely controlled dose. The result is that the dose administered to the subject with such a device is not a well-controlled dose.
An apparatus which is capable of administering a closely controlled dose of an aerosol would be desirable such as for use in preliminary drug studies to determine, for example, whether a sufficient amount of an aerosol form of a new drug is absorbed through the lungs for that to be an efficacious route of administration. In studies such as these, the ability to administer a very closely controlled dose of the drug is important in order for measured blood levels of the drug and the like to be meaningful.